I've been in this situation in reverse, I knew things about my mum that I couldn't tell my dad. I still haven't told him, but in hindsight, I wish I had done what curiosity and skyep have suggested because I mightn't have harboured so much anger towards my mother if she had come clean and taken the burden from me, she still doesn't admit to it even though I know for sure what was happening.
We get on well now, but it has been a long, tough road. Try not to end up getting involved, no matter how much you feel for your mum, which is natural, you have a father too, and I doubt very much he intended to hurt you or drag you into this. Tell him how it is affecting you and what you know, put the ball in his court and let him deal with the guilt for a little while and let him see how it feels. I really, really feel for you honety, it's brought back a lot of memories for me and trust me I know how hard it is for you, so don't let it eat you up and make sure you take time to make sure YOU are coping, and not spend all your time making sure your parents (and / or siblings) are ok too. Let us know how it goes X X